1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a liquid crystal display device of a vertical alignment mode.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for various kinds of equipment such as personal computers, OA equipments, and TV sets because the display devices have many advantages such as lightness, compactness and low power consumption. In recent years, the liquid crystal display device has also been used in mobile terminal equipments such as a mobile phone, a car navigation device and a game player.
The liquid crystal display devices include thin-film transistors (TFTs) as switching elements of pixels using semiconductor layers, such as amorphous silicon film and poly-silicon film. The thin-film transistors are formed on an optically transparent substrate. Recently, the thin-film transistors made from the poly-silicon film for driver circuits have been also formed on the same substrate as that for the switching transistors because the mobility of the poly-silicon by electric field effect is greater than that of the amorphous silicon. Since the substrate, however, is made of a glass substrate, which is optically transparent and electrical insulating, static electricity is charged on the substrate. The static electricity may destroy the switching transistors and/or the driver circuits to significantly lower a yield of the liquid crystal display device.
In the case, particularly, that a gate insulation film is formed in a small polycrystalline silicon pattern on the substrate, that long gate electrode leads are formed on the gate insulation film, and that electric capacitors are formed between the polycrystalline silicon patterns and the gate electrode leads, electrostatic destruction may occur between the polycrystalline silicon pattern and the gate electrodes. In other words, it is highly possibly for the static electricity to destroy the thin-film transistors.
Japanese laid open patent application 2005-134446 discloses a construction to suppress such problem. In the Japanese laid open application, gate electrode lines formed on a glass substrate are separated and connected by a connecting element for respective pixels to shorten lengths of the gate electrode lines. When the substrate is charged with static electricity, the increase in voltages between the gate electrode lines and the switching elements are effectively suppressed so that electrostatic destruction of the switching elements can be prevented.
Generally, wiring layers used in the liquid crystal display device are desired to have a low resistance characteristic. Accordingly, the wiring layers are formed of electrically conductive material such as aluminum, molybdenum and tungsten. However, since the above conductive materials are not optically transparent, portions of the pixels on which the wiring layers are formed are optically shielded, which results in decrease of brightness of the display. In the technology disclosed in the Japanese laid open patent application, the gate electrode lines are arranged in line so as to cross the pixels and are divided into two segments in the pixel. The segments are connected by a connecting element. The connecting element is designed to have a larger width than the segments in consideration of a margin to accommodate a mask shift during a manufacturing process. The connecting element with a large width results in decrease of an aperture ratio which influences the brightness of the display due to increase of the shield area by the connecting element.
On the other hand, in order to obtain a high quality display, further characteristics such as a wide viewing angle and a high contrast ratio are requested in the liquid crystal display device. Recently, a Multi-Domain type liquid crystal display device using Multi-Domain Vertical Alignment (MVA) mode in which a pixel includes a plurality of domains, has been used as shown in Japanese patent application 2008-197493. In this type liquid crystal display device, the wide viewing angle is achieved by the plurality of domains. Further, a high contrast ratio which results in more complete black display is obtained because a retardation of the liquid crystal material becomes nearly zero. The liquid crystal molecules around an alignment film are aligned in a horizontal direction with respect to the substrate by adopting a homeotropic alignment treatment.